The present invention relates generally to a method and apparatus for the manufacture of a continuous film or web of thermoplastic material which film or web includes portions having a thickness which exceeds the thickness of adjoining portions of the film or web. The invention also relates to a web of thermoplastic material manufactured by means of the method and apparatus of the present invention.
By a method which is well known within the technique of package manufacture, hollow bodies of plastic material can be manufactured by deep-drawing or compression moulding of portions of a plastic material heated to softening. Such a method unfortunately brings with it a certain unavoidable wastage, since not the whole web can be utilized for the deep-drawing. What remains of the plastic material web after certain parts of it have been used for the manufacture of hollow bodies is a network which is formed by the portions which are located between the deep-drawn portions of the packing material web. This network can be used again in many cases for example, by having it ground down and melted again, but it has been found that such a reuse on the one hand can be expensive, whilst on the other hand the properties of the material, e.g. colour, smell, taste are altered after a number of reuses, consequently, the process of repeatedly reusing the material cannot be carried on for too long, and on certain periodically occurring occasions old wastage has to be discarded, and a new start has to be made with a plastic material web of new material from which the wastage once more can be used.
As indicated above, the wastage constitutes a problem in the manufacture of deep-drawn hollow parts from a plastic material web, but in accordance with the present invention it is possible to manufacture a continuous film or web of thermoplastic material in a manner which causes the wastage to be substantially reduced. The basic idea is that the plastic material web along the portions which are used in the deep-drawing of the said hollow bodies has a thickness which is adapted to the size and ultimate wall thickness of the deep-drawn container. Furthermore portions of the plastic material web adjoining them, which are not used of in the deep-drawing, are made appreciably thinner. Since the deep-drawing for the most part takes place with a number of moulding tools working over the width of the plastic material web it is necessary that the web be held together and that portions intended for deep-drawing, which have greater material thickness, are arranged with relatively great accuracy in relation to one another and in such a manner that each portion intended for deep-drawing can be brought into engagement with the respective moulding tool. The portions of the plastic material web located in between, which thus constitute a wastage, have the function of holding together the portions intended for deep-drawing in a predetermined pattern and at a mutual predetermined distance corresponding to the positioning of the moulding tool. Accordingly, the wastage portions in between cannot be made arbitrarily thin but must have enough rigidity to retain the thicker portions intended for deep-drawing in their respective predetermined position.
The invention, as mentioned above, relates to an arrangement for the manufacture of a continuous film or web which is intended to be deep-drawn for the formation of hollow bodies with each body separately or in combination with other bodies forming packing containers. The apparatus of the present invention includes an extruder for supplying molten plastic material. A metal body is connected to the extruder including on the one hand a duct, through which the plastic mass discharged from the extruder is arranged to be conveyed and distributed, and on the other hand two circular-cylindrical seat surfaces, which are arranged to co-operate with two rotatable driven rolls. One of the rolls has recessed portions whose shape, size and mutual position correspond to the said thickened portions of the film or web produced.